The fiduciary duty is a legal relationship between two or more parties, most commonly a "fiduciary" or "trustee" and a "principal" or "beneficiary", that in English common law is arguably the most important concept within the portion of the legal system known as equity. Since the Judicature Acts merged the courts of Equity (historically based in England's Court of Chancery) with the courts of common law, the concept of fiduciary duty also became usable in common law courts.